content - CSS: Cascading Style Sheets 编辑

The content CSS property replaces an element with a generated value. Objects inserted using the content property are anonymous replaced elements.

/* Keywords that cannot be combined with other values */
content: normal;
content: none;

/* <image> values */
content: url("http://www.example.com/test.png");
content: linear-gradient(#e66465, #9198e5);

/* alt text for generated content, added in the Level 3 specification */
content: url("http://www.example.com/test.png") / "This is the alt text";

/* values below can only be applied to generated content using ::before and ::after */

/* <string> value */
content: "prefix";

/* <counter> values, optionally with <list-style-type> */
content: counter(chapter_counter);
content: counter(chapter_counter, upper-roman);
content: counters(section_counter, ".");
content: counters(section_counter, ".", decimal-leading-zero);

/* attr() value linked to the HTML attribute value */
content: attr(value string);

/* Language- and position-dependent keywords */
content: open-quote;
content: close-quote;
content: no-open-quote;
content: no-close-quote;

/* Except for normal and none, several values can be used simultaneously */
content: open-quote counter(chapter_counter);

/* Global values */
content: inherit;
content: initial;
content: unset;

Syntax

Values

none

The pseudo-element is not generated.

normal

Computes to none for the ::before and ::after pseudo-elements.

<string>

Specifies the "alt text" for the element. This value can be any number of text characters. Non-Latin characters must be encoded using their Unicode escape sequences: for example, \000A9 represents the copyright symbol.

<image>

An <image>, denoted by the <url>() or <gradient> data type, or part of the webpage, defined by the element() function, denoting the content to display.

<counter>()

The value of a CSS counter, generally a number produced by computations defined by <counter-reset> and <counter-increment> properies. It can be displayed using either the counter() or counters() function.

The counter() function has two forms: 'counter(name)' or 'counter(name, style)'. The generated text is the value of the innermost counter of the given name in scope at the given pseudo-element. It is formatted in the specified <list-style-type> (decimal by default).

The counters() function also has two forms: 'counters(name, string)' or 'counters(name, string, style)'. The generated text is the value of all counters with the given name in scope at the given pseudo-element, from outermost to innermost, separated by the specified string. The counters are rendered in the indicated <list-style-type> (decimal by default).

attr(x)

The value of the element's attribute x as a string. If there is no attribute x, an empty string is returned. The case-sensitivity of attribute names depends on the document language.

open-quote | close-quote

These values are replaced by the appropriate string from the quotes property.

no-open-quote | no-close-quote

Introduces no content, but increments (decrements) the level of nesting for quotes.

Accessibility concerns

CSS-generated content is not included in the DOM. Because of this, it will not be represented in the accessibility tree and certain assistive technology/browser combinations will not announce it. If the content conveys information that is critical to understanding the page's purpose, it is better to include it in the main document.

• Accessibility support for CSS generated content – Tink
• Explanation of WCAG, Guideline 1.3 – MDN
• Understanding Success Criterion 1.3.1 | W3C Understanding WCAG 2.0

Formal definition

Formal syntax

normal | none | [ <content-replacement> | <content-list> ] [/ <string> ]?
where 
<content-replacement> = <image>
<content-list> = [ <string> | contents | <image> | <quote> | <target> | <leader()> ]+
where 
<image> = <url> | <image()> | <image-set()> | <element()> | <paint()> | <cross-fade()> | <gradient>
<quote> = open-quote | close-quote | no-open-quote | no-close-quote
<target> = <target-counter()> | <target-counters()> | <target-text()>
<leader()> = leader( <leader-type> )
where 
<image()> = image( <image-tags>? [ <image-src>? , <color>? ]! )
<image-set()> = image-set( <image-set-option># )
<element()> = element( <id-selector> )
<paint()> = paint( <ident>, <declaration-value>? )
<cross-fade()> = cross-fade( <cf-mixing-image> , <cf-final-image>? )
<gradient> = <linear-gradient()> | <repeating-linear-gradient()> | <radial-gradient()> | <repeating-radial-gradient()> | <conic-gradient()>
<target-counter()> = target-counter( [ <string> | <url> ] , <custom-ident> , <counter-style>? )
<target-counters()> = target-counters( [ <string> | <url> ] , <custom-ident> , <string> , <counter-style>? )
<target-text()> = target-text( [ <string> | <url> ] , [ content | before | after | first-letter ]? )
<leader-type> = dotted | solid | space | <string>
where 
<image-tags> = ltr | rtl
<image-src> = <url> | <string>
<color> = <rgb()> | <rgba()> | <hsl()> | <hsla()> | <hex-color> | <named-color> | currentcolor | <deprecated-system-color>
<image-set-option> = [ <image> | <string> ] <resolution>
<id-selector> = <hash-token>
<cf-mixing-image> = <percentage>? && <image>
<cf-final-image> = <image> | <color>
<linear-gradient()> = linear-gradient( [ <angle> | to <side-or-corner> ]? , <color-stop-list> )
<repeating-linear-gradient()> = repeating-linear-gradient( [ <angle> | to <side-or-corner> ]? , <color-stop-list> )
<radial-gradient()> = radial-gradient( [ <ending-shape> || <size> ]? [ at <position> ]? , <color-stop-list> )
<repeating-radial-gradient()> = repeating-radial-gradient( [ <ending-shape> || <size> ]? [ at <position> ]? , <color-stop-list> )
<conic-gradient()> = conic-gradient( [ from <angle> ]? [ at <position> ]?, <angular-color-stop-list> )
<counter-style> = <counter-style-name> | symbols()
where 
<rgb()> = rgb( <percentage>{3} [ / <alpha-value> ]? ) | rgb( <number>{3} [ / <alpha-value> ]? ) | rgb( <percentage>#{3} , <alpha-value>? ) | rgb( <number>#{3} , <alpha-value>? )
<rgba()> = rgba( <percentage>{3} [ / <alpha-value> ]? ) | rgba( <number>{3} [ / <alpha-value> ]? ) | rgba( <percentage>#{3} , <alpha-value>? ) | rgba( <number>#{3} , <alpha-value>? )
<hsl()> = hsl( <hue> <percentage> <percentage> [ / <alpha-value> ]? ) | hsl( <hue>, <percentage>, <percentage>, <alpha-value>? )
<hsla()> = hsla( <hue> <percentage> <percentage> [ / <alpha-value> ]? ) | hsla( <hue>, <percentage>, <percentage>, <alpha-value>? )
<side-or-corner> = [ left | right ] || [ top | bottom ]
<color-stop-list> = [ <linear-color-stop> [, <linear-color-hint>]? ]# , <linear-color-stop>
<ending-shape> = circle | ellipse
<size> = closest-side | farthest-side | closest-corner | farthest-corner | <length> | <length-percentage>{2}
<position> = [ [ left | center | right ] || [ top | center | bottom ] | [ left | center | right | <length-percentage> ] [ top | center | bottom | <length-percentage> ]? | [ [ left | right ] <length-percentage> ] && [ [ top | bottom ] <length-percentage> ] ]
<angular-color-stop-list> = [ <angular-color-stop> [, <angular-color-hint>]? ]# , <angular-color-stop>
<counter-style-name> = <custom-ident>
where 
<alpha-value> = <number> | <percentage>
<hue> = <number> | <angle>
<linear-color-stop> = <color> <color-stop-length>?
<linear-color-hint> = <length-percentage>
<length-percentage> = <length> | <percentage>
<angular-color-stop> = <color> && <color-stop-angle>?
<angular-color-hint> = <angle-percentage>
where 
<color-stop-length> = <length-percentage>{1,2}
<color-stop-angle> = <angle-percentage>{1,2}
<angle-percentage> = <angle> | <percentage>

Examples

Headings and quotes

This example inserts quotation marks around quotes, and adds the word "Chapter" before headings.

HTML

<h1>5</h1>
<p>According to Sir Tim Berners-Lee,
  <q cite="http://www.w3.org/People/Berners-Lee/FAQ.html#Internet">I was
    lucky enough to invent the Web at the time when the Internet
    already existed - and had for a decade and a half.</q>
  We must understand that there is nothing fundamentally wrong
  with building on the contributions of others.
</p>

<h1>6</h1>
<p>According to the Mozilla Manifesto,
  <q cite="http://www.mozilla.org/en-US/about/manifesto/">Individuals
    must have the ability to shape the Internet and
    their own experiences on the Internet.</q>
  Therefore, we can infer that contributing to the open web
  can protect our own individual experiences on it.
</p>

CSS

q {
  color: blue;
}

q::before {
  content: open-quote;
}

q::after {
  content: close-quote;
}

h1::before  {
  content: "Chapter ";  /* The trailing space creates separation
                           between the added content and the
                           rest of the content */
}

Result

Image combined with text

This example inserts an image before the link. If the image is not found, it inserts text instead.

HTML

<a href="http://www.mozilla.org/en-US/">Mozilla Home Page</a>

CSS

a::before {
  content: url("https://mozorg.cdn.mozilla.net/media/img/favicon.ico") / " MOZILLA: ";
  font: x-small Arial, sans-serif;
  color: gray;
}

Result

Targeting classes

This example inserts additional text after special items in a list.

HTML

<h2>Paperback Best Sellers</h2>
<ol>
  <li>Political Thriller</li>
  <li class="new-entry">Halloween Stories</li>
  <li>My Biography</li>
  <li class="new-entry">Vampire Romance</li>
</ol>

CSS

.new-entry::after {
  content: " New!";  /* The leading space creates separation
                        between the added content and the
                        rest of the content */
  color: red;
}

Result

Images and element attributes

This example inserts an image before each link, and adds its id attribute after.

HTML

<ul>
  <li><a id="moz" href="http://www.mozilla.org/">
    Mozilla Home Page</a></li>
  <li><a id="mdn" href="/wiki/">
    Mozilla Developer Network</a></li>
</ul>

CSS

a {
  text-decoration: none;
  border-bottom: 3px dotted navy;
}

a::after {
  content: " (" attr(id) ")";
}

#moz::before {
  content: url("https://mozorg.cdn.mozilla.net/media/img/favicon.ico");
}

#mdn::before {
  content: url("https://www.wenjiangs.com/wp-content/uploads/2020/mozilla/mdn-favicon16.png");
}

li {
  margin: 1em;
}

Result

Element replacement

This example replaces an element's content with an image. You can replace the contents of an element with either a <url>() or an <image> value. Content added with ::before or ::after will not be generated as the contents of the element have been replaced.

HTML

<div id="replaced">Mozilla</div>

CSS

#replaced {
  content: url("https://mdn.mozillademos.org/files/12668/MDN.svg");
}

#replaced::after { /* will not show if element replacement is supported */
  content: " (" attr(id) ")";
}

Result

Specifications

Browser compatibility

BCD tables only load in the browser

See also

• Replaced elements
• ::after
• ::before
• ::marker
• quotes
• url() function